Pre-formed powder delivery to powder press machine

ABSTRACT

Methods for fabricating an interconnect for a fuel cell system that include forming a metal powder into a preform structure, positioning the preform structure in a die cavity of a press apparatus, and compressing the preform structure in the press apparatus to form the interconnect. Further embodiments include use of thin inserts in the die cavity to provide reduced permeability and/or including filler material in the die cavity.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/501,572, filed Sep. 30, 2014, which claims benefit of priority toU.S. Provisional Patent Application No. 61/885,048, filed Oct. 1, 2013,the entire contents of which are incorporated herein by reference.

BACKGROUND

In a high temperature fuel cell system, such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow canbe a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol,or methanol. The fuel cell, operating at a typical temperature between750° C. and 950° C., enables the transport of negatively charged oxygenions from the cathode flow stream to the anode flow stream, where theion combines with either free hydrogen or hydrogen in a hydrocarbonmolecule to form water vapor and/or with carbon monoxide to form carbondioxide. The excess electrons from the negatively charged ion are routedback to the cathode side of the fuel cell through an electrical circuitcompleted between anode and cathode, resulting in an electrical currentflow through the circuit.

In order to optimize the operation of SOFCs, the oxidizing and fuelflows should be precisely regulated. Therefore, the flow regulatingstructures, such as interconnects in the fuel cell system should beprecisely manufactured.

SUMMARY

Embodiments include methods for fabricating an interconnect for a fuelcell system that include forming a metal powder into a preformstructure, positioning the preform structure in a die cavity of a pressapparatus, and compressing the preform structure in the press apparatusto form the interconnect.

In various embodiments, the preform structure may be formed withsufficient structural integrity to maintain its shape while the preformstructure is positioned in the die cavity, and may be designed to breakapart during compressing in the press apparatus. The preform structuremay be made with or without a lubricant or organic binder.

In embodiments, a thickness of the preform structure is varied toprovide a substantially uniform compaction ratio for regions of theinterconnect having different thicknesses. The thickness of the preformstructure may adjusted (e.g., by adding or removing powder) in selectportions of the structure to control a density of a corresponding regionof the interconnect. Critical regions of the interconnect may thereby beformed with higher density.

Further embodiments include methods for fabricating an interconnect fora fuel cell system that include providing a metal powder and at leastone insert in a die cavity of a powder press apparatus, and compressingthe metal powder and the insert in the press apparatus to form theinterconnect, wherein the at least one insert provides reducedpermeability in at least a portion of the interconnect. The at least oneinsert may be a sheet of a non-porous material, such as a metal foil.The at least one insert may be embedded within the pressed interconnectand/or over a surface of the interconnect.

Further embodiments include methods for fabricating an interconnect fora fuel cell system that include providing a metal powder mixed with atleast one filler material in a die cavity of a powder press apparatus,compressing the metal powder and the filler material in the powder pressapparatus to form the interconnect. The filler material may be a ceramicmaterial (e.g., alumina) and/or a pore forming material.

Further embodiments include methods for fabricating an interconnect fora fuel cell system that include providing powder to a die cavity of apress apparatus, vibrating the powder into a desired shape in the diecavity, and compressing the powder in the press apparatus to form or theinterconnect or a preform structure of the interconnect.

Further embodiments include interconnects formed in accordance with theabove-described methods and systems configured to fabricateinterconnects in accordance with the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate example embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the features of theinvention.

FIG. 1A illustrates a side cross-sectional view of a SOFC stack.

FIGS. 1B and 1C show, respectively, top and bottom views of aninterconnect for a SOFC stack.

FIGS. 2A and 2B are respective side cross sectional and top views of apowder metallurgy (PM) apparatus for making interconnects for a fuelcell stack.

FIGS. 3A and 3B are respective side cross sectional and top views of aprior art PM apparatus.

FIG. 4 is a side cross-sectional schematic view of a powder pressapparatus with a powder preform structure being placed within a diecavity of the apparatus.

FIG. 5A is a side cross-sectional view of metal powder loaded into a diecavity using a conventional fill shoe.

FIG. 5B is a side cross-sectional view of a powder preform structurehaving a varying thickness.

FIG. 6 is a schematic illustration of a continuous feed system forforming a powder preform structure for delivery to a powder pressapparatus.

FIG. 7 is a side cross-sectional schematic view of a powder pressapparatus and metal powder having foil inserts being loaded into the diecavity of the apparatus.

FIG. 8 is a side cross-sectional schematic view of a powder pressapparatus and metal powder having filler material being loaded into thedie cavity of the apparatus.

FIGS. 9A-9F are side cross-sectional schematic views of a powder pressapparatus used in a powder pressing method of another embodiment.

FIGS. 10A-10E are photographs of steps in a method of making aninterconnect according to an embodiment.

FIGS. 11A-11D are perspective views from varying angles of a mask withmovable scrapers for pushing powder.

FIGS. 12A-12C are cross-sectional views of a shoe with adjustable heightscrapers.

FIGS. 13A-13B are perspective views of a vibrator shaping tool assembly.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawing. Wherever possible, the same reference numberswill be used throughout the drawing to refer to the same or like parts.References made to particular examples and implementations are forillustrative purposes, and are not intended to limit the scope of theinvention or the claims.

FIG. 1A illustrates a SOFC stack in which each SOFC 1 comprises acathode electrode 7, a solid oxide electrolyte 5, and an anode electrode3. Fuel cell stacks are frequently built from a multiplicity of SOFC's 1in the form of planar elements, tubes, or other geometries. Fuel and airhas to be provided to the electrochemically active surface, which can belarge.

The gas flow separator 9 (referred to as a gas flow separator plate whenpart of a planar stack), containing gas flow passages or channels 8between ribs 10, separates the individual cells in the stack. The gasflow separator plate separates fuel, such as a hydrocarbon fuel, flowingto the fuel electrode (i.e. anode 3) of one cell in the stack fromoxidant, such as air, flowing to the air electrode (i.e. cathode 7) ofan adjacent cell in the stack. At either end of the stack, there may bean air end plate or fuel end plate (not shown) for providing air orfuel, respectively, to the end electrode.

Frequently, the gas flow separator plate 9 is also used as aninterconnect which electrically connects the anode or fuel electrode 3of one cell to the cathode or air electrode 7 of the adjacent cell. Inthis case, the gas flow separator plate which functions as aninterconnect is made of or contains electrically conductive material.FIG. 1 shows that the lower SOFC 1 is located between two interconnects9.

FIGS. 1B and 1C show, respectively, top and bottom views of aninterconnect 9. The portions of interconnect 9 shown in sidecross-section in FIG. 1A are provided along lines A-A in FIGS. 1B and1C. The interconnect 9 contains gas flow passages or channels 8 betweenribs 10. The interconnect 9 in this embodiment includes at least oneriser channel 16 a for providing fuel to the anode-side of the SOFC 1,as illustrated by arrow 29. The riser channel 16 a generally comprises afuel inlet riser opening or hole that extends through at least one layerof the fuel cells and interconnects in the stack. As illustrated in FIG.1C, the fuel can flow through the inlet riser channel 16 a to theanode-side of each fuel cell. There, the fuel can collect in an inletplenum 17 a (e.g., a groove in the interconnect's surface), then flowover the fuel cell anode 3 through gas flow channels 8 formed in theinterconnect 9 to an outlet plenum 17 b and then exit through a separateoutlet riser channel 16 b.

The cathode side, illustrated in FIG. 1B, can include gas flow passagesor channels 8 between ribs 10 which direct air flow 44 over the cathodeelectrode of the fuel cell. Seals 15 a, 15 b can seal the respectiverisers 16 a, 16 b on the cathode-sides of the interconnect and fuel cellto prevent fuel from reaching the cathode electrode of the fuel cell.The seals may have a donut or hollow cylinder shape as shown so that therisers 16 a, 16 b extend through the hollow middle part of therespective seals 15 a, 15 b. The seals 15 a, 15 b can include a elevatedtop surface for contacting against the flat surface of the adjacent SOFC1. A peripheral seal 15 c can seal the anode-sides of the interconnectand fuel cell to prevent air from reaching the anode electrode of thefuel cell.

For solid oxide fuel cell stacks, the interconnect 9 is typically madefrom an electrically conductive metal material, and may comprise achromium alloy, such as a Cr—Fe alloy made by a powder metallurgytechnique. The powder metallurgy technique may include pressing andsintering a Cr—Fe powder, which may be a mixture of Cr and Fe powdersand/or pre-alloyed Cr—Fe powder, to form a Cr—Fe alloy interconnect in adesired size and shape (e.g., a “net shape” or “near net shape”process). A typical chromium-alloy interconnect may comprise at leastabout 80% chromium, and preferably more than about 90% chromium, such asabout 94-96% (e.g., 95%) chromium by weight. The interconnect maycontain less than about 20% iron, and preferably less than about 10%iron, such as about 4-6% (e.g., 5%) iron by weight. The interconnect maycontain less than about 2%, such as about zero to 1% of other materials,such as yttrium or yttria, as well as residual or unavoidableimpurities.

In a conventional method for fabricating interconnects, blended Cr andFe elemental powders are pressed in a hydraulic or mechanical press toproduce a part having the desired interconnect shape. The Cr and Fepowders are blended with an organic binder and pressed into so-called“green parts” using a conventional powder metallurgy technique. The“green parts” have substantially the same size and shape as the finishedinterconnect (i.e., “near net shape”). The organic binder in the greenparts is removed before the parts are sintered. The organic binder isremoved in a debinding process in a furnace that is operated atatmospheric pressure at a temperature of 400° C. to 800° C., preferablyin a reducing environment, such as under flow of hydrogen gas. Afterdebinding, the compressed powder Cr—Fe interconnects are sintered athigh-temperature (e.g., 900-1550° C.) to promote interdiffusion of theCr and Fe. The interconnects may undergo a separate controlled oxidationtreatment, such as by exposing the interconnects to an oxidizingambient, such as air at high temperature after sintering, prior to useof the interconnects in the stack.

Powder metallurgy (PM) technology creates the shape of a part usingthree components in a compaction press—the upper punch 402 a, the lowerpunch 402 b, and a die 404. The design of the interconnect necessitatesvarious cross sectional thickness to be molded by features on thepunches, i.e., there is cross sectional thickness variation in thedirection of compaction tonnage (FIGS. 2A and 2B). This is differentfrom most parts that are processed using PM technology where the punches402 a, 402 b are typically flat and the die 404 is the component thatcontains the geometric features, i.e., the cross sectional thickness inthe direction of compaction tonnage is uniform (FIGS. 3A and 3B).

Various embodiments include improved methods of fabricatinginterconnects using powder metallurgy. In one embodiment, at least aportion of the metal powder is formed into a powder preform structurethat is loaded into a die cavity of a powder press apparatus, and thepowder is compressed to form the interconnect. The preform structure mayhave a size and shape that provides a desired compaction ratio of thepowder in the die cavity to the pressed powder interconnect, independentof the topography of the upper and lower punches of the powder pressapparatus. In embodiments, the compaction ratio may be substantiallyuniform across the interconnect. In some embodiments, the preformstructure may be provided with extra powder in regions corresponding tocritical regions of the final interconnect in order to provide higherdensity in these critical regions. In a non-limiting embodiment, thelower and/or upper surface of the preform is not flat and containsprotrusions and depressions, and/or the preform has thicker and thinnerparts (i.e., it has a non-uniform thickness along its length).Embodiments of the method may also provide a significant increase inthroughput of a powder press apparatus by reducing the powder deliverytime and/or eliminating the requirement of a separate powder deliveryapparatus (e.g., a “fill shoe”).

FIG. 4 schematically illustrates a powder preform structure 401 beingpositioned within a die cavity 406 of a powder press apparatus 400. Inorder to press a part with sufficient density throughout the part,typically the powder within the die 404 must be strategicallydistributed before pressing the part. In a conventional process forfabricating pressed powder interconnects, this is typically accomplishedby a device adapted for delivering the powder to the die 404 (i.e., a“fill shoe,” not shown in FIG. 4), and/or by swiping the deliveredpowder (i.e., removing powder from select regions of the die 404). Inthe embodiment illustrated in FIG. 4, at least a portion of the powderin the die 404 is formed into a preform structure 401 (which may also bereferred to as a powder “patty” or “cookie”) having sufficientstructural integrity to maintain its shape while it is positioned withinthe cavity 406 of the powder press die 404. This may enable a much moreprecise and reproducible fill of the die 404. The shape of the preformstructure 401 is not necessarily close to the shape of the finalinterconnect (i.e., it is not “net shape” or “near net shape”), and maylack fine features of the final interconnect, such as the individualribs 8 and channels 10 defining the flow field(s) of the interconnect,inlet and outlet plenums 17 a, 17 b, and/or flat elevated surfaces onwhich the seals 15 a-c are placed (see FIGS. 1A-C). Fine features of theinterconnect may be defined by the upper and lower punches 402 a, 402 bduring compaction of the powder. In embodiments, the powder preformstructure 401 may have a varying thickness in different regions of thestructure 401 which allows the amount of powder in the correspondingregions of the final pressed interconnect to be more effectivelycontrolled. A material or fabrication process used to form the preformstructure 401 (e.g., glue, organic binder, etc.) may be strong enough toallow the structure 401 to be transported into the press 400, but weakenough (e.g. brittle, so it breaks down in the initial moment ofcompression) not to interfere with the flow of the powder duringpressing of the part. U.S. Pat. No. 8,173,063 discloses a double pressmethod for forming an interconnect. However, the first press forms aninterconnect which is different from a pre-form structure 401 because itdoes not collapse into powder during the second pressing step as doesthe pre-form structure 401 of the present embodiments. Also, the firstpress in the double press technique defines the individual fine features(e.g., ribs, channels, etc.) of the finished interconnect in exaggeratedform, whereas the pre-form structure 401 may lack such fine features. Inembodiments, a lubricant (e.g., organic binder) which is typically usedto form the powder pressed “green part,” as described above, may alsoserve the function of (temporarily) adhering the powder in a preformstructure 401 for loading into the press, and may be, for example, awax. In other embodiments, as described below, the powder preformstructure 401 may be formed without the use of a lubricant or separatebinder or bonding agent. A variety of materials and technique can beapplied to create the initial shape (e.g., pressing or compacting powderwith or without further additives).

Embodiments may overcome the inherent imbalance in the powderdistribution when the shoe fills the die cavity to a relatively flatprofile. The reason is that in conventional powder press systems, thepowder delivery shoe traverses the die cavity, depositing the availablepowder with a flat topography (i.e., a flat upper surface of the filledpowder). The lower punch can be actuated to move up or down during thisfill to have some control of the amount of powder left by the shoe, butthat still has limitations in the distribution in a transverse directionto the direction of the shoe travel.

The powder delivery by the shoe is a significant (30%) portion of thecycle time to compact powder into a part. In the embodiment using thepreform structure, it will be seen that the delivery can be effectedmuch more quickly, reducing the cycle time, and increasing the number ofparts made per minute.

In embodiments, a method of fabricating an interconnect for a fuel cellincludes making a shaped pre-form structure 401 of the powder, where thepowder volume is adjusted to be what is needed for the final part. Thismay include making adjustments for regions of higher or lower density inthe final interconnect, as well as to accommodate features of the die404, such as core rod(s) or the inner walls of the die 404. Adjustingfor higher or lower density in the finished interconnect may beaccomplished by adding or subtracting powder from regions of thepre-form structure 401 corresponding to the desired regions of highand/or low density in the final interconnect. At the core rod or diewall, the pre-form structure 401 may be configured to be offset slightlyleaving a small gap (e.g., 0.1-1.0 mm, such as 0.1-0.5 mm, includingabout 0.3 mm) between the edges of the preform structure 401 and theside walls and core rods of the die 404, in order to allow clearanceduring delivery of the pre-form structure 401 into the die 404.Additional powder may be provided at the locations proximate to the corerods and die edges (e.g., the thickness of the preform structure 401 maybe increased proximate to the gaps) to provide sufficient powder to fillthe gap regions during compaction.

The powder may be formed into a preform structure 401 without lubricant,as described above. For metal powder without lubricant, the powder willstick together at a reasonable pressure, leaving a “cookie” of thepre-formed powder available for delivery to the press. The preformstructure 401 may be formed and maintained at room temperature (e.g.,20-23° C.). When the preform structure 401 is fabricated with alubricant (e.g., wax), higher pressure may be needed to keep the preformin the desired shape. An alternative includes cooling the powdered metaland lubricant to increase the viscosity of the lubricant so that thepreform structure does not deform before it is delivered to the mainpowder press tool. This may enable the powder to be formed into apreform structure 401 using lower pressure and less expensive equipment.Similarly, a lubricant that is more viscous at room temperature may beused to enable the preform structure to be made less expensively atlower pressures.

The preform structure 401 may be formed in a different die pressapparatus than the apparatus 400 used to form the interconnect.

FIG. 5A schematically illustrates the profile of a metal powder 500after loading into a die cavity using a conventional “fill shoe.” Asshown in this figure, the shoe fills the die cavity to a relatively flatprofile (i.e., with a flat upper surface). The compaction ratio for apowder press apparatus having upper and lower punches is the ratio ofthe initial powder thickness (T in FIG. 5A) to the thickness (t) of thefinal component after compaction. Many powder pressed parts, such asinterconnects, have non-uniform thicknesses. This is indicated by thedashed line in FIG. 5A, which schematically illustrates the finalthickness (i.e., t₁, t₂, etc.) of different regions of an interconnect9. For example, referring to FIGS. 1A-C, the flat elevated surfaces ofan interconnect 9 on which the window seals 15 c and donut seals 15 a,15 b sit may have a first thickness, while the regions corresponding tothe fuel and air flow fields may have a second, reduced thickness,particularly where the flow fields have an offset configuration, suchthat the ribs 8 on the anode-facing side of the interconnect are alignedwith the channels 10 on the cathode-facing side, and vice versa.Additional regions of the interconnect 9, such as fuel inlet and outletplenums 17 a, 17 b, may have an even smaller cross-sectional thickness.

For components with non-uniform thickness, the compaction ratio (T/t)will vary across the part, as shown in FIG. 5A. If the compaction ratiois not equal across the part, then the powder would need to flowlaterally (i.e., transverse to the direction of compaction) to even thisout and provide an interconnect having a relatively uniform density.However, the friction is so high that the powder can only move smallamounts laterally during compaction.

In various embodiments, a powder preform structure 401 as describedabove may be configured to provide a substantially uniform compactionratio (T/t) across the interconnect. In embodiments, a thickness, T, ofthe preform structure 401 may be varied to substantially correspond tothickness variations in the final pressed interconnect. Thus, as shownin FIG. 5B, the preform structure 401 may have a first thickness, T₁, ina peripheral region 504 which corresponds to the periphery of the finalinterconnect having a thickness t₁, and a second reduced thickness, T₂,in an interior region 506 which corresponds to the flow field region(s)of the final interconnect having a reduced thickness t₁. Put anotherway, the powder preform structure 401 may be essentially a verticallyexpanded example of the thickness map of the final pressed part. Asubstantially uniform compaction ratio (T/t) may be obtained over theinterconnect.

A surface 502 of the preform structure 401 (e.g., the bottom surface inFIG. 5B) may have a relatively flat profile. This surface 502 may beplaced facing up in the die cavity of the powder press apparatus, andadditional powder (if needed) may be deposited over the surface 502 ofthe preform structure 401 using conventional techniques (e.g., a powderdelivery shoe). The additional powder may be deposited as a layer havinga uniform thickness.

In another embodiment, the preform structure 401 may also enable moreeffective control of the density of the final interconnect. For example,as shown in FIG. 5B, the thickness of the preform structure 401 may beincreased in regions corresponding to critical areas of theinterconnect, such as in the areas surrounding the fuel holes (e.g.,fuel inlet and outlet riser openings) 16 a, 16 b (see FIGS. 1B-C). Asshown in FIG. 5B, the preform structure 401 has an increased thickness,T₃, in region 508 (i.e., the thickness T₃ in region 508 is greater thanis required to provide the desired uniform compaction ratio). The use ofregions with extra powder relative to what would be required for auniform compaction ratio allow extra density in these regions. This canremove the requirement for the powder metallurgy chamfer which is usedto make sure the interconnect is leak tight. A powder metallurgy chamfermay refer to a region of extra compaction along an edge of a part, suchas the inner edge of the holes (e.g., fuel holes) in a pressed powderinterconnect. The punches of the powder press apparatus may include asmall protrusion that provides the extra compaction along the edge(s) ofthe part, forming the chamfer feature and reducing the permeability ofthe part. However, these small protrusions can break off or deform,particularly when a high velocity compaction is employed, which canlimit the lifetime of the punch to unacceptable levels. By providingexcess powder in critical areas of the interconnect, such as surroundingthe fuel holes, the protrusions used to form the chamfer may beeliminated, thus improving the useful life of the punch.

The peripheral equipment used to make the preform structure 401 mayinclude an apparatus (e.g., a small press, roller, etc.) that isconfigured to form the preform structure into the desired shape, and adevice to deliver the preform structure to the main powder pressapparatus while maintaining its shape. A small press may be a lowertonnage press, which is much less expensive than a high tonnage press400 used to press the interconnect into net or near net shape. Forpowder with lubricant, a refrigerator to cool the powder and pre-formstructure may be used to keep the preform structure in one piece.

FIG. 6 illustrates an embodiment of a continuous feed system 600 forforming a powder preform structure 401 for delivery to a powder pressapparatus 400. An initial quantity of powder may be deposited using afirst powder deposition apparatus 602. The powder may be deposited as alayer having a uniform thickness and flat topology. The deposited powdermay then be fed (e.g., using a belt or other conveying mechanism) to afirst compaction apparatus 604 (as indicated by the dashed arrow in FIG.6). The compaction apparatus 604 may include a roller or wheel that isconfigured to compact the powder to form an initial pre-form structure.A second powder deposition apparatus 606 may then selectively depositadditional quantities of powder in one or more areas corresponding toregions of increased thickness and/or increased density requirements inthe final interconnect. The additional powder may be compacted by anadditional compaction apparatus 608. This process may repeat until thepreform structure 401 has a desired thickness profile. The preformstructure 401 may then be inserted into the powder press apparatus 400using an automated handling system 610 (e.g., a robotic system), and thepreform structure 401 may be compacted (optionally with additionalpreformed powder structure(s) and/or loose powder) to form theinterconnect. Advantages of this embodiment include high-throughput andreduced cost due to the relatively low cost of the compaction apparatus(e.g., wheels 604, 606) used to form the powder preform structure. Inembodiments, the separate fill shoe for the main powder press apparatus400 may be eliminated.

Further embodiments include methods of fabricating an interconnect usingpowder metallurgy that include providing a thin, non-porous insertwithin the die cavity with the metal powder stock and pressing the metalpowder and the insert to form the interconnect. FIG. 7 schematicallyillustrates a plurality of inserts 701, 703 being placed with metalpowder within a die cavity 406 of a powder press apparatus 400. Theinserts 701, 703 may each comprise a continuous sheet of metal foil thatextends generally transverse to the direction of compaction. In theembodiment of FIG. 7, a first foil insert 701 extends over substantiallythe entire “footprint” of the die cavity 406 and includes powder bothabove and below the insert 701. After powder pressing, the first insert701 is embedded within the pressed powder interconnect. A pair ofsmaller inserts 703 are provided over portions of a surface of thepowder stock. After pressing, the inserts 703 are bonded to a surface ofthe interconnect. The powder may comprise one or more preform structures401, as described above, and the insert(s) 701, 703 may be embeddedwithin the preform structure 401, placed on a surface of a preformstructure 401, or positioned between preform structures 401, asappropriate. Alternatively, the insert(s) 701, 703 may be placed on,below and/or embedded within loose powder within the die cavity 406(e.g., the die may be partially filled with powder, insert 701 may beplaced over the powder followed by additional powder delivery, andinsert(s) 703 may be placed on top of the powder prior to compaction).

A pressed metal powder interconnect should have sufficient density toseparate the fuel from the oxidizer and not let the fuel attack the sealfrom “underneath” (i.e., from within the interconnect). Currently, thisdensity requirement is achieved by optimizing (e.g. maximizing) thedensity of the pressed part and optionally by pre-oxidizing the part toreduce the remaining porosity.

Applicants have discovered that an interconnect need not be completelygas impervious throughout its entire thickness and thus the requirementsof high density and low porosity may be relaxed. Separation of fuel andoxidizer has to be only good enough not to affect the performance of thedevice. If some fuel and oxidizer mix (react) fuel will be lost(reduction in efficiency) and heat will be generated (which will have tobe removed). If the amount of gas reacting is small (enough) the impactof performance may become negligible.

One way to obtain the desired degree of gas impermeability (i.e.,hermeticity) in an interconnect is to include prefabricated componentsin the pressing process. In one embodiment as shown in FIG. 7, one ormore inserts 701 (e.g., a thin foil) with the same size/shape as thefootprint of the interconnect may be provided in the die cavity 406along with the powder to be pressed. The insert 701 may slightly deformduring pressing but may provide a hermetic barrier within the finalinterconnect. If there is a hermetic barrier within the interconnect,the remainder of the interconnect can be left more porous. This allowsuse of less material and/or lower compression forces (cheaper tools,larger part, etc.). This may also eliminate the need for pre-oxidationof the interconnect. Pre-oxidation also serves to obtain a compatiblecoefficient of thermal expansion (CTE), which can be alternativelyrealized with a modified (optionally cheaper) powder mixture.

Another sensitive area within the interconnect are the seal areas wherefuel has to be kept from attacking the seals from within theinterconnect. Alternatively or in addition to the insert 701 describedabove, one or more additional inserts 703 (e.g. foils) can be placedinto the die cavity, above or below the powder, in regions correspondingto seal areas of the interconnect (e.g., toroidal regions of theinterconnect surrounding the fuel risers 16 a, 16 b which support thetoroidal or “donut” shaped seals 15 a, 15 b on the air side of theinterconnect as shown in FIG. 1B). After pressing, the insert(s) 703 arebonded to the pressed powder interconnect and serve as the sealingsurface in these regions of the final interconnect. In this way,increased gas impermeability (i.e., hermeticity) may be realized in thesealing surface in regions under seals 15 a, 15 b.

The inserts 701, 703 (e.g., foils) may be made from the same material asthe powder used for pressing or from a different material (e.g., Cr—Fealloy). If the insert 701, 703 is thin enough a certain degree of CTEmismatch between powder and the insert is permissible.

The inserts 701, 703 as described above may be utilized in combinationwith the powder pre-formed structure 401 (e.g., “patties” or “cookies”)as described above in connection with FIGS. 4-6.

In embodiments, multiple powder preform structures 401 and/or inserts701, 703 with different properties may layered in the die cavity beforepressing (or a combination of lose powder and preform structures/insertsmay be positioned in the die cavity) to provide a functionally gradedcomponent. Not every layer has to fulfill all functions. A small layersatisfying hermeticity may be sufficient to separate air and fuel.

FIG. 8 schematically illustrates an additional embodiment of fabricatingan interconnect using powder metallurgy in which filler material 801 isincluded in the metal powder stock. The cost of chromium is a costdriver for conventional interconnects. Applicants have discovered thatit is possible to maintain functionality of the interconnect whilesubstituting part of the Cr/Fe powder with other materials, includingnon-metal materials such as ceramics. For example alumina may be used asa filler material. In embodiments, up to about 8% by weight (e.g., 3-6%by weight) of the metal powder stock may be substituted by a fillermaterial, such as alumina and/or other ceramic materials. The fillermaterial may be in powder form that is mixed with the metal powdermixture, and may be included in a powder preform structure as describedabove. The filler material (e.g., alumina) may have a different CTE thanthe metal powders (e.g., Cr—Fe) and would therefore alter the CTE of thepressed part. This can be corrected by adjusting the composition of thebase metal powder (e.g., modifying the amount of Cr and Fe in the powderuntil the mixture produces the desired CTE). The filler may be selectedsuch that it can survive the pressing and the sintering process.

In some embodiments, all or a portion of the filler material maycomprise pore formers. In other words a filler material is used thatleaves behind voids (pores). In general this is undesirable and maycause functional problems, but if the pore formers are applied locally(in areas of the interconnect where porosity is acceptable) or ifcombined with inserts that provide sufficient hermeticity, as describedabove, pore formers may be utilized. Pore formers may be organicparticles which turn to gas and escape during sintering/debindering toleave voids.

Selective inclusion of pore formers may also extend the functionality ofthe interconnect. Internal flow passages may be created, and pores inthe interconnect may enhance catalysis of fuels.

Because interconnects are being designed thinner and thinner, theirdensity and topographical uniformity are becoming more sensitive to theshape of the metal powder prior to compaction into an interconnect.Further embodiments, illustrated in FIGS. 9A-9F, provide complexinterconnect shapes in a high throughput production environment.

As shown in FIG. 9A, a preform structure may be formed by providing themetal powder in the die cavity of a powder press apparatus and shapingthe metal powder with an upper shaping punch to form the metal powderinto the preform structure positioned in the die cavity of the pressapparatus. In steps 1 and 2 shown in FIGS. 9A and 9B, the metal powder901 can be placed in a die cavity 406 formed by a lower tool (e.g.,lower punch 402 b) and an outer mold frame (e.g., die 404). In step 3,the metal powder 901 may be shaped by pressing the upper shaping punch902 into the metal powder. In some embodiments, shaping the metal powderwith the upper shaping punch may comprise of vibrating at least part ofthe powder press apparatus and/or rotating at least part of the powderpress apparatus. The shape of the upper shaping punch 902 can bedetermined to shape the metal powder into the preform 401 shape bymeasuring the topography of the shaped powder, and designing the uppershaping tool to match this shape.

In an embodiment, shaping the metal powder 901 with the upper shapingpunch 902 may be followed by removing the upper shaping punch 902,inserting the compaction punch 402 a into the die 404 cavity 406, andcompressing the preform structure 401 with the upper compaction punch inthe powder press apparatus to form the interconnect 9, as illustrated instep 4 of FIG. 9D.

A further embodiment may allow for the control of the compaction ratioby ensuring that the shape of the preform 401 is different from theshape of the interconnect. In an embodiment, at least one channel 10 abetween ribs 8 a in the preform 401 is deeper than at least one channel8 between ribs 10 in the interconnect 9. As illustrated in FIGS. 9E and9F, respectively, least one protrusion 904 a in the upper shaping punch902 is longer than at least one corresponding protrusion 904 b in theupper compression punch 402 a. Thus, the upper shaping punch 902 is adifferent punch from the upper compaction punch 402 a. The lower punch402 b may be the same punch or different punches during the shaping stepin FIG. 9C and the compaction step in FIG. 9D.

The embodiments described above improve interconnect characteristics bydecoupling metal powder dispensation from shaping. These embodimentsimprove process simplicity and reduce cost of equipment. Theseembodiments also provide relatively complex powder shaping capability ina relatively short amount of time.

Non-uniform density distribution after compaction can create majorissues in interconnects. For example, density variation can create weakspots on the interconnect, increasing the risk that the interconnectwill become permeable to the separate fuel and air. Density variationcan also cause the interconnect to reach the highest density at thethinnest region, inhibiting the feasibility of improving density inother areas of the interconnect. The following embodiments can achievecomplex three dimensional powder shaping to provide accurate powdercontrol during the compaction process to promote control during thecompaction process.

In one embodiment illustrated in FIGS. 10A-10E, an insert may be usedduring the powder fill process to substitute powder volume, and finalpowder shape is achieved after the insert is vertically removed. Theamount of powder removal can be controlled by the thickness of theinsert, and the insert design can achieve complex powder shapingcontours to accommodate the desired thickness variation of interconnectgeometry. Forming a metal powder into a preform structure may compriseproviding a lower portion of the metal powder in the die cavity of apowder press apparatus, providing an insert covering at least part ofthe lower portion of the metal powder, providing an upper portion of themetal powder on the lower portion of the metal powder, and removing theinsert vertically from the metal powder to form a desired metal powdershape. An embodiment may further comprise moving the lower tooling 402 bof the cavity 406 to adjust cavity volume for the lower portion of themetal powder and/or the upper portion of the metal powder.

FIGS. 10A-10E illustrate one embodiment in which the inserts 1002 a,1002 b may be used to achieve complex powder shaping contours. First,the lower tooling (e.g., lower punch 402 b) may be moved to create aninitial cavity 406 for powder fill, as illustrated in FIG. 10A. Next,the lower portion 901 a of the metal powder may be provided from thefill shoe 1004 to fill the cavity 406, as shown in FIG. 10B. The lowerpunch 402 b is moved down and one or more inserts 1002 a, 1002 b arethen placed upon the lower portion 901 a of the powder, as shown in FIG.10C. In this embodiment, the inserts 1002 a, 1002 b have the inverseshapes of the plenums 17 a, 17 b shown in FIG. 1C. The inserts may haveany other suitable shape depending on the interconnect shape. Then,additional powder (e.g., the upper portion 901 b of the metal powderfrom the fill shoe 1004) is provided on the initial powder fill 901 aand inserts, as shown in FIG. 10D. Finally, FIG. 10E illustrates apowder shape achieved after the inserts are vertically removed. Sincethe inserts 1002 a, b have the inverse shapes of the plenums 17 a, 17 b,removal of the inserts results in recesses 1017 a, 1017 b in the powderthat correspond to the plenums which will be formed in the interconnectafter compaction. In an additional embodiment, the process shown inFIGS. 10A-10D may be repeated any number of times before removing theinserts to achieve more complex multiple level thickness in the powdershaping. This embodiment is compatible with standard powder fill shoe1004 operations. Additionally, it allows complex powder shaping to beachieved in a single process step with the insert design.

In another embodiment, complex three dimensional powder shaping can beachieved by providing the metal powder in the die cavity of the powderpress apparatus and selectively vacuuming the metal powder from one ormore desired locations. The amount of removal volume can be achieved byvacuum process parameter control (e.g., pressure and/or distance of avacuum nozzle from the powder). In an embodiment, a dedicated vacuumnozzle or manifold design is based on powder mixture characteristics(e.g., size, density, and/or viscosity). In an embodiment, the finalpowder shaping can be achieved by mounting the vacuum nozzle on an XYZlinear stage and programming the moving path, distance, and speed. Thenozzle may thus move around the powder surface to create arbitrarypowder shaping.

In an exemplary aspect of the present invention, the lower tooling(e.g., lower punch 402 b) can be located in this apparatus to create acavity 406 corresponding to the largest thickness of the powder 901 tobe filled into the cavity 406. After filling the cavity with metalpowder using the fill shoe described above, a motion control linearstage fixture mounted with a vacuum nozzle is operated with apre-programmed moving path to create powder shape with multiplethickness steps and a complex contours. In a further exemplaryembodiment, the vacuum nozzle can be connected to a vacuum hose. Toimprove to throughput of this process, a vacuum manifold may be utilizedto apply multiple vacuums simultaneously at plural desired regions toform the powder shape in a single operation. Embodiments using a vacuumto remove metal powder have a non-limiting advantage of accomplishingcomplex three dimensional powder shaping in just a single powder filloperation. Additionally, the program controlled moving path of thepowder removal vacuum provides the flexibility to achieve differentpowder shapes without changing any hardware. They further provide closedloop control on detailed powder shaping tuning based on compactionresults.

In another embodiment, complex three dimensional powder shaping can beachieved by providing the metal powder in the die cavity of a powderpress apparatus, providing at least one mask and at least one scraperover the metal powder, and operating the at least one scraper to removeexcess metal powder. FIGS. 11A-11C illustrate one non-limiting mask andscraper design. FIG. 11A shows a perspective view of a mask and scraperassembly overview. FIG. 11B illustrates a bottom view of the mask andscraper assembly and FIG. 11C illustrates atop view of the mask andscraper assembly of FIG. 11A. FIG. 11D is a close-up view of the scraperof FIG. 11C.

As illustrated in FIGS. 11A-11D, the mask and scraper assembly may bepressed into the powder located on the support. The excess powder thenrises through openings 1102 a, 1102 b in the mask 1101. Scrapers 1103may move on an axis or optionally rails 1104 to push powder away fromopenings 1102 a, 1102 b and onto ledge 1105 of the opening in the mask.The scrapers can be separate from the mask and can be operated manuallyor mechanically. Further, the scrapers can be any blade or movingsurface which can move powder (e.g., the scraper can be a manual wiperblade).

In an exemplary embodiment, the fill shoe described above can fill thecavity with powder above the lower punch 402 b. The mask 1101 can thenbe installed and the scraper(s) 1103 operated on the pre-defined movingpath to shift and move excess powder into non-critical locations. Themask and scraper may then be removed, and a vacuum applied to remove theexcess powder from the mask.

In one embodiment, the scraper may shift excess powder into anoncritical designated location 1105 outside the powder fill/toolingregion of the mask surface, such as the ledge 1105 next to the openings1102 in the mask 1101. A vacuum may then be employed to remove theexcess powder from the ledge 1105. In an embodiment, the mask definesthe moving path of the scrapers 1103 a, 1103 b, thereby generating thedesired powder shape and also protecting the non-excess powder. Thedistance the scraper is submerged in the metal powder may be adjustable,thereby controlling an amount of metal powder removed.

Exemplary mask openings 1102 may be located in locations correspondingto plenums 17 a, 17 b in the interconnect 9. The scrapers 1103 a, 1103 bremove the excess powder in the plenum locations such that the plenumrecesses described above are formed in the interconnect after thepressing step.

The mask and scraper method allows for complex three dimensional powdershaping in just a single powder fill operation. Further, it minimizesconcerns of mixture uniformity and achieves a large pattern shaping areain a single operation.

In another embodiment, complex three dimensional powder shaping can beachieved by providing a programmable linear array of adjustable heightscrapers attached to the powder fill shoe or another support and raisingand lowering the programmable linear array of adjustable height scrapersas the powder fill shoe or another support is retracted, therebyremoving excess metal powder. The adjustable height scrapers may beprogrammed to move up and down to correspond to the desired final powdershape as the shoe is retracted. In a preferred embodiment, the lineararray is wide enough to extend the full extent of the powder area (e.g.,the interconnect 9 width) being prepared for powder pressing.

FIGS. 12A-12C illustrate a programmable linear array of adjustableheight scrapers 1203 attached to a powder delivery fill shoe 1004. FIG.12A is a side view of a shoe 1004 with the scrapers 1203 attached to theforward tip. The powder is stored in the middle shoe section 1204 beforemoving forward across the die cavity. The vertical arrow shows thevertical motion of the individual scrapers and the horizontal arrowshows the retraction movement of the fill shoe 1004 over the powder 901.An optional feed tube 1206 is provided for additional powder as thepowder is released through the lower section of the shoe 1004. Scrapers1203 can be any surface or blade which can move powder. For example, thescrapers 1203 can be lowered in location of the plenums 17 a, 17 b onthe interconnect 9 to leave recesses in these locations when the shoeretracts. The scrapers may be raised over other portions of the powder901 which correspond to ridges or ribs in the interconnect 9.

FIG. 12B illustrates a front-side view of the fill shoe 1004 shown inFIG. 12A. In FIG. 12B, the individual scrapers 1203 are shown in fulldown position. FIG. 12C illustrates a front view of the same shoe 1004.However, unlike FIG. 12B, FIG. 12C illustrates the shoe 1004 in adifferent horizontal location over the powder 901 where seven totalindividual scrapers 1203 a, b, c are displaced upwards (i.e., raised) bya controlled amount, to allow more powder to be left behind as the shoeretracts back over the powder 901. The remaining scrapers 1203 d remainin full down position.

Utilizing adjustable height scrapers offers several advantages,including creating complex three dimensional powder shaping in just asingle powder fill operation. Further, it minimizes concerns of mixtureuniformity and achieves a large pattern shaping area in a singleoperation. Additionally, it provides for a large degree of flexibilityto change the powder shape, since it is under programmed control of acontroller. This method also permits quick feedback if the powderdensity is slightly different from what is normal or typical. In anotherembodiment, the scrapers 1203 are mounted on another support (e.g., asupport plate) rather than on the shoe 1004.

While solid oxide fuel cell interconnects, end plates, and electrolyteswere described above in various embodiments, embodiments can include anyother fuel cell interconnects, such as molten carbonate or PEM fuel cellinterconnects, or any other metal alloy or compacted metal powder orceramic objects not associated with fuel cell systems.

An embodiment also provides for a method of forming an interconnect withor without the use of a preform structure (i.e., the preform structureis optional). In an embodiment, powder may be provided into the diecavity 406 and may then be vibrated and compressed to form the preformstructure 401 or the final interconnect 9. For example, the uppershaping punch 902 may also serve as a vibration assembly for vibratingthe powder, as illustrated in FIG. 13A. The punch 902 contains avibrator 1301 located above the bottom punch surface 1302 and below thetop portion 1304.

It may also be desirable to change the relative vertical position of thevarious regions (e.g., protrusions 904 a and/or edge surfaces) on thebottom of the upper shaping punch 902. This may be accomplished withadjusting guides 1303. FIG. 13B shows that adjusting guides 1303 may beraised or lowered to adjust the adjustable shaping surfaces 1305 (i.e.,portions which protrude into and/or out of bottom punch surface 1302)which correspond to the protrusions 904 a in FIG. 9E and/or the edgesurfaces of the lower surface of the punch 902. The adjustable shapingsurfaces 1305 may form one or more plenums on the interconnect 9, suchas inlet plenum 17 a and outlet plenum 17 b, shown in FIG. 1C, aftercompression of the powder. Specifically, the adjusting guides 1303 maybe controlled to push the adjustable shaping surfaces 1305 up or downthrough the bottom surface 1302 of punch 902. Having adjustable, asopposed to fixed, surfaces may be preferable because it may be desirableto adjust the shape of the bottom punch surface 1302 based on the powderdensity in order to maintain a substantially constant compaction ratio(e.g., the compaction ratio illustrated in FIG. 5B). While the upperpunch is described above, the vibrator and/or adjustable surfaces mayalso be provided in the lower punch in addition to or in place of theupper punch.

It may also be preferable to determine the optimal time to stopvibrating the powder (i.e., “end point”). If left vibrating longer thannecessary, powder may be vibrated into undesirable locations. Forexample, powder may crawl up the gap between the punch 902 and the die404. Additionally, excessive vibration times may lead to detrimentalsegregation of the powder particles.

An embodiment provides determining the end point based upon measuringthe change in the vibration of the powder as the powder moves intoposition. Any suitable method of measuring vibration may be used, forexample by optically or acoustically detecting amplitude and/orfrequency of the powder vibration in the cavity as the powder moves intoposition.

Acoustically, the sound of the powder vibration may be detected using anacoustic detector and a processor which can compare the output value ofthe acoustic detector to values in a look up table. Optically, a lasermay be used to detect powder vibration frequency and/or amplitude. Thelaser beam may be pointed at the powder through an opening with a clearcover in die 404. The vibrating powder reflects the laser light to aphotodetector through the same or a different opening. A processor maybe used to compare the output value of the photodetector to values in alook up table.

The powder vibration changes in amplitude and/or frequency as the powderstops flowing laterally into position in the cavity. As the end point ofthe flow is approached, the powder vibration amplitude decreases and thefrequency increases. When the powder stops flowing laterally intoposition, the bottom of the vibrator shaping tool assembly is in contactwith substantially uniformly dense powder and there is no furthersubstantial change in the powder vibration frequency or amplitude. Thevibration may be stopped when the end point of the powder flow has beenreached when the frequency and amplitude no longer substantially change(e.g., the change is less than 5%) over time (e.g., between 10 sec and10 minutes). Vibration may be stopped when the vibration frequencyreaches 5-5000 Hz (e.g., 35-45 Hz) and remains substantially constant(e.g., varies less than 5% over time). Alternatively, the vibration maybe stopped when the vibration amplitude reaches the average size of anindividual powder particle and remains substantially constant (e.g.,varies less than 5% over time). This may occur in the when the amplitudeis 0.02-2 mm (e.g., 0.1-0.2 mm).

Embodiments which determine the end point may be used in conjunctionwith other embodiments. For example, an end point may be detected forvibrating the preform structure 401. Vibrating the preform structure maythus be stopped when the end point is reached.

The foregoing method descriptions are provided merely as illustrativeexamples and are not intended to require or imply that the steps of thevarious embodiments must be performed in the order presented. As will beappreciated by one of skill in the art the order of steps in theforegoing embodiments may be performed in any order. Words such as“thereafter,” “then,” “next,” etc. are not necessarily intended to limitthe order of the steps; these words may be used to guide the readerthrough the description of the methods. Further, any reference to claimelements in the singular, for example, using the articles “a,” “an” or“the” is not to be construed as limiting the element to the singular.

Further, any step of any embodiment described herein can be used in anyother embodiment. The preceding description of the disclosed aspects isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these aspects will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other aspects without departing fromthe scope of the invention. Thus, the present invention is not intendedto be limited to the aspects shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method of fabricating an interconnect for afuel cell system, comprising: forming a metal powder into a preformstructure; positioning the preform structure in a die cavity of a pressapparatus; and compressing the preform structure in the press apparatusto form the interconnect.
 2. The method of claim 1, wherein forming themetal powder into the preform structure and positioning the preformstructure in the die cavity of the press apparatus comprises: providingthe metal powder in the die cavity of the press apparatus; and shapingthe metal powder with an upper shaping punch to form the metal powderinto the preform structure positioned in the die cavity of the pressapparatus.
 3. The method of claim 2, wherein shaping the metal powderwith the upper shaping punch comprises pressing the upper shaping punchinto the metal powder.
 4. The method of claim 3, wherein shaping themetal powder with the upper shaping punch further comprises at least oneof: vibrating at least part of the press apparatus; and rotating atleast part of the press apparatus.
 5. The method of claim 2, furthercomprising: removing the upper shaping punch; and compressing thepreform structure with an upper compaction punch in the press apparatusto form the interconnect.
 6. The method of claim 2, wherein a shape ofthe preform is different from a shape of the interconnect.
 7. The methodof claim 6, wherein at least one channel between ribs in the preform isdeeper than at least one channel between ribs in the interconnect. 8.The method of claim 6, wherein at least one protrusion in the uppershaping punch is longer than at least one corresponding protrusion inthe upper compression punch.
 9. A method of fabricating an interconnectfor a fuel cell system, comprising providing powder to a die cavity of apress apparatus; vibrating the powder into a desired shape in the diecavity; and compressing the powder in the press apparatus to form theinterconnect or a preform structure of the interconnect.
 10. The methodof claim 9, wherein: vibrating the powder into a desired shape comprisesvibrating the powder to fill the die cavity; and compressing the powdercomprises compressing the powder to form the interconnect.
 11. Themethod of claim 10, further comprising adjusting a height of protrusionson at least one punch in the press apparatus as a function of powderdensity prior to compressing the powder using the punch.
 12. The methodof claim 10, further comprising: detecting at least one of a frequencyor an amplitude of powder vibration, and stopping the vibration when thefrequency or the amplitude of the vibration substantially ceaseschanging over time.
 13. The method of claim 12, wherein the frequency orthe amplitude is detected using a laser.
 14. The method of claim 12,wherein the frequency or the amplitude is detected acoustically.
 15. Themethod of claim 12, further comprising stopping the vibration when theamplitude is substantially equal to an average particle size of thepowder.
 16. The method of claim 12, wherein the vibration is stoppedwhen the frequency is between 5 Hz and 5000 Hz.
 17. The method of claim12, wherein vibration is stopped when the amplitude is between 0.01 mmand 2 mm.